Device for automatically compensating failure of a lamp

ABSTRACT

A device for automatically compensating failure of a lamp in which voltage drop across a series resistor is applied to the emitter of a transistor for detecting failure of a lamp and the forward voltage drop across a diode is applied to the base of said transistor to restrain the voltage drop across the series resistor, and the main current of a transistor circuit controlled by the failure detecting transistor is made to flow through a resistor to form a positive feedback to said failure detecting transistor within the range controlled by said diode and resistor thereby to further lower the voltage drop across the series resistor. The failure detecting transistor is sufficiently turned on by the above-mentioned circuit arrangement to energize a backup lamp.

United States Patent Morita 1 May 13,1975

Appl. No.: 384,533

Inventor:

Assignee:

[30] Foreign Application Priority Data Aug. 2, i972 Japan 47-90364 Sept. 8, 1972 Japan 47-104230 US. Cl. 315/88; 3l5/93; 340/251 Int. Cl. H05b 39/10; HOSb 4l/46 Field of Search 3l5/88, 93; 340/46, 25!

References Cited UNITED STATES PATENTS 3,076,l23 l/l963 McDermott 3l5/88 3,562.580 2/l97l Blomgren BIS/88 FOREIGN PATENTS OR APPLICATIONS 954 l l6 2/l963 United Kingdom 3l5/38 Primary Examiner-Stanley D. Miller, Jr Attorney, Agent, or Firm-Woodhams, Blanchard and Flynn [57} ABSTRACT A device for automatically compensating failure of a lamp in which voltage drop across a series resistor is applied to the emitter of a transistor for detecting failure of a lamp and the forward voltage drop across a diode is applied to the base of said transistor to restrain the voltage drop across the series resistor, and the main current of a transistor circuit controlled by the failure detecting transistor is made to flow through a resistor to form a positive feedback to said failure detecting transistor within the range controlled by said diode and resistor thereby to further lower the voltage drop across the series resistor. The failure detecting transistor is sufficiently turned on by the abovementioned circuit arrangement to energize a back-up lamp.

9 Claims, 3 Drawing Figures SHEU 10F 2 m b nM U T Q 7 2 m. g? il F 2 1 A f L. m D R a R ll illl lLl B u MD C w 2 L 4 W m 5 2 k 6 3 w m. 1 rm M F i D m PATENTEU HAY 1 319. 5 3 p 883 777 SHEET 20F 2 FIG. 3

S} fi DEVICE FOR AUTOMATICALLY COMPENSATING FAILURE OF A LAMP This invention relates to a device for automatically compensating a possible failure or burning-out of a lamp such as a headlamp of an automobile, which is capable of detecting failure of the lamp and energizing a spare or back-up lamp for the lamp burnt out upon detection of the failure.

Hitherto. there have been proposed many devices for detecting a possible failure of a lamp by the voltage drop across a resistor connected in series with the lamp in which said voltage drop across the resistor is applied between the base and the emitter ofa PNP type transistor to bias said transistor into a conducting state as long as the lamp has been in the normal condition.

These devices, however. involve some problem that power loss in the circuit is considerably large due to the necessity that the voltage drop across the resistor is to be larger than the threshold voltage (about 0.6V) of the transistor.

One way to solve the above-mentioned problem is to apply the voltage drop across the series resistor to the emitter of a transistor and the forward voltage drop across a diode to the base of said transistor thereby to enable reduction of the voltage drop across the series resistor. However, this way also has a disadvantage that sufficient base current is not obtained and several amplifiers are additionally required.

This invention was made to overcome the abovementioned disadvantages and therefore it is an object of the present invention to provide a device for automatically compensating failure ofa lamp which is capable of supplying sufficient base current with a reduced number of amplifiers thereby to simplify the circuit arrangement of the device and enable the device to be manufactured at a reasonable cost.

It is another object of the present invention to provide a device for automatically compensating a failure of a lamp which is capable of minimizing a power loss owing to its circuit arrangement in which the voltage drop across a series resistor is adapted to just slightly lower the potential at an emitter.

It is a further object of the present invention to provide a device for automatically compensatingfailure of a lamp which is capable of reducing the number of scmiconductivc elements of large capacity to be connccted because of its circuit arrangement in which a main current for energizing a spare lamp is adapted to flow through one semiconductivc element thereby to reduce the manufacturing cost.

According to the present invention. there is provided a device for automatically compensating failure in a lamp, which comprises said lamp; a back-up lamp which is adapted to be energized in place of said lamp when said lamp is burnt out; a series resistor connected between a power supply and said lamp. a PNP-typc transistor having an emitter connected between said series resistor and said lamp. a collector connected to a base of another transistor. but of NPN-typc. as men tioned below. and a base connected to the power supply through another rcsistor and a diode; and the NPN- type transistor connected to be turned on when the potential at the emitter ol said PNP-typc transistor is higher than the potential at the base thereof to further lower the potential at the base thereof; said back-up lamp being adapted to be energized upon the turning on of said NPN-type transistor.

The invention will be better understood from the following description taken in connection with the accom panying drawings in which;

FIG. 1 is a circuit diagram of one form ofa device for automatically compensating failure of a lamp and em bodying the present invention;

FIG. 2 is a similar circuit diagram of another form of the device according to the present invention; and

FIG. 3 is a similar circuit diagram of a further form of the device according to the present invention.

Referring now to FIG. 1, there is illustrated one embodiment of the present invention. B is a power supply such as a battery mounted on an automobile. S is a power supply switch such as a lighting switch, L is a load such as a headlamp of an automobile, L is a spare or backup lamp which is adapted to be compensatingly energized upon failure of the lamp L PL is a pilot or alarm lamp for indicating a failure of the lamp L,. and R is a resistor connected in series with the lamp L which causes a certain voltage drop when the lamp L is energized. Numeral l designates a reference voltage circuit composed of a diode D and resistors R and R wherein there is a relation R R and the current flowing in the forward direction of the diode D is determined by the resistors R and R and the diode D to keep the forward voltage drop at point a about 0.5V. Numeral 2 designates a control circuit comprising a PNP-type transistor Tr for detecting failure of the lamp. an NPN-type power transistor Tr: and a resistor R which control circuit 2 is adapted to compensatingly light the back-up lamp L when the lamp L, is burnt out or disconnected.

The operation of the thus constructed device will be explained in the following.

The silicon transistor is normally in a nonconducting state when the base-emitter voltage is about 0.3V. begins to be conductive when said voltage is about 0.5V and is completely turned on when said voltage is about 0.7V.

When the lamp L operates properly. if the voltage drop across the series resistor R is determined to be 0.2V (the electric potential at point b will be then lower than the power voltage by 0.2V) and the electric poten tial at point a (the juncture of the resistors R and R is determined to be 0.5V lower than the power voltage. a voltage of only 0.3V is applied across the base and the emitter of the transistor Tr keeping said transistor Tr, turned off. The transistor Tr: is accordingly kept in a nonconducting state. As a result. there will be supplied no collector current through the diode D of the reference voltage circuit l. letting both the back-up lamp L and the warning lamp Pl. remain dcencrgi7ed.

In the event of a failure of the lamp l... the voltage drop across the series resistor R ceases and the potential difference of 0.5V appearing at u is applied across the base and the emitter of the transistor Tr,.

Then, the transistor Tr is biased into a conducting state. permitting a collector current to flow. whereby the potential at the base of the transistor Tris raised and said transistor Tr is in turn biased into a conducting state. Since the collector current of the transistor lr flows through the diode D and the resistor R the potential at point a is lowered front the predetermined voltage and the base bias of the transistor Tr, is increased aceordingly. deepening the conducting of said transistor Tr,. which in turn further biases said transistor Tr, to be conductive. Thus, the switching of the transistor Tr, can be well sped up. In this connection, if the resistor R, is selected to make the potential at point a lower than the power voltage by 0.7 0.8V when the transistors Tr, and Tr, are turned on, said transistors Tr, and Tr, become conductive and the backup lamp L, and the alarm lamp PL are lit without a power loss due to the transistor Tr Though in the operation as mentioned above the connection of the transistors Tr, and Tr, functions equivalently to a silicon controlled rectifier, the present circuit arrangement controls the positive feedback through the transistor Tr by the potential at point a which is determined by the diode D, and the resistor R so that the transistors Tr, and Tr, become turned off when the failure of the lamp L, is eliminated and it returns to the normal condition with a voltage drop across the series resistor R,.

Another embodiment of the present invention will be explained referring to FIG. 2. This embodiment is iden tical with the first embodiment except a control circuit 2 which is composed of transistors Tr, to Tr, and resistors R,, to R,.

In the operation, when the lamp L, is burnt out or disconnected and the transistor Tr, is started to be con ductive, the base current is supplied to the transistor Tr, through the resistor R biasing said transistor Tr into a conducting state, lowering the potential at point a through the resistor R,, and supplying the base current to the transistor Tr, through the resistor R,. In other words, the emitter of the transistor Tr is grounded to make a positive feedback to a through the resistor R,,, thereby to facilitate the control of such a feedback. The power loss by the transistor Tr, can be reduced by employing a PNP-type transistor in which the voltage drop across the collector and the emitter thereof is smaller as compared with an NPN-type transistor. The transistor Tr, is protected from an inrush (high starting transient) current to be supplied to the back-up lamp L, and the alarm lamp PL by regulating the base current with the resistor R,.

A further embodiment of the present invention is shown in FIG. 3 wherein numeral 1 designates a lamp energizing circuit including an NPN-type transistor Tr, and a collector resistor R,, of said transistor Tr, and numeral 2 designates a failure detecting circuit comprising a resistor R, connected in series with a lamp L,, a PNP-type transistor Tr, for detecting a failure of the lamp. a diode D, for applying a base bias to said transistor Tr,,, a collector resistor R,, of said transistor Tr,,. a base protective resistor R,, of the transistor Tr, and a bias resistor R,,,

The operation of the present embodiment is as follows.

When the lamp L, operates properly, a current is supplied, upon closing of a swtich S, to the lamp L, through the resistor R, to energize said lamp L,. When the current flows through the resistor R,, there is caused a voltage drop across said resistor R, to lower the potential at the emitter of the transistor Tr,,. The base bias applied to said transistor Tr is determined by a voltage divider formed of the resistor R,,, the diode D and the resistor R,,,. The base potential is substantially equal to the power source potential substracted by the voltage drop across the diode D and the voltage drop across the resistor R,, can be disregarded because the circuit current is extremely small.

If the voltage drop across the resistor R, is determined to be 0.3V and the voltage drop across the diode D to be 0.6V, there appears a difference of only 0.3V in potential, which is not sufficient to turn on the transistor Tr,,. Accordingly, both of said transistor Tr and the transistor Tr, are kept in a nonconducting state and the back-up lamp is not energized.

Upon failure or burning-out of the lamp L,. there is supplied no current through the resistor R, and no voltage drop across said resistor R,, so that the emitter of the transistor Tr, is maintained at the power supply voltage. On the other hand, to the base of the transistor Tr, is applied 0.6V due to the voltage drop across the diode D thus making the difference in potential between the emitter and the base be 0.6V and biasing said transitor Tr, into a conducting state.

As a result, the collector voltage is raised to the power supply voltage and the base current is supplied to the transistor Tr, through the base protective resistor R,, biasing said transistor Tr, into a conducting state. When the transistor Tr, begins to be turned on. there occurs a voltage drop across the resistor R,, which 'deepens the base bias of the transistor Tr, thereby to completely turn on said transistor Tr, and the transistor Tr, is turn.

Upon turning on of the transistor Tr,, the back-up lamp L is energized for the burnt out lamp L, and the alarm lamp PL is lit to annunciate the failure of the lamp to the driver and prevent possible trouble due to such a failure.

The potential at the base of the transistor Tr, is substantially equal to the power supply potential if the base protective resistor R is not connected. The potential at the emitter thereof is 0.6 to 0.8V lower than the base potential and the potential at the collector is 0.3V lower than said base potential due to the voltage drop (0.3V) across the resistor R,, so that the potential difference between the collector and the emitter is restrained within 0.3 to 0.5\/. Thus, the resistor R,, acts to reduce the power loss in the transistor Tr,

Even when the resistor R,, is selected to be large and there is caused a large voltage drop thereacross, the problem of having the base bias of the transistor Tr become too large does not occur. Stated illustratively, through the resistor R,,, there flow the base current of the transistor Tr and the current from the diode D by which the base bias is applied to the transistor Tr,,. so that a large voltage drop across the resistor R,, would result simply in reluctant flow of the current from the diode D, and cause no lowering of the base potential. The resistor R,, finally controls the base current of the transistor Tr,

The following operation will be seen when the failure of the lamp is eliminatead and it begins to operate properly. In the transistor Tr, turned on upon the failure of the lamp L,. the base-emitter voltage is about 0.9V. Said base-emitter voltage of the transistor Tr, is lowered not more than 0.6V because the voltage drop across the resistor R, in the steady-state current of the lamp L, is 0.3V so that said transistor Tn, is not turned off. However, the inrush (turn-on transient) current flows through the lamp L, increasing the voltage drop across the resistor R, to level more than in the steady state, whereby said transistor Tn, is turned off easily. The transistor Tr, is in turn deenergized to extinguish the back-up lamp and the lamp L, remains energized.

According to the present invention, as mentioned above, voltage drop across a series resistor is applied to the emitter of a transistor for detecting failure of a lamp and forward voltage drop across a diode is applied to the base of said transistor to restrain the voltage drop across the series resistor, and a main current of a transistor circuit controlled by the failure detecting transistor is made to flow through a resistor to form a positive feedback to said failure detecting transistor within the range controlled by said diode and resistor thereby to further lower the voltage drop across the series resistor. The failure detecting transistor is sufficiently turned on by the above-mentioned circuit arrangement to energize a back-up lamp.

As the voltage drop across the series resistor just slightly lowers the emitter voltage of the transistor, power loss can be minimized. Further according to the present invention, possible fluctuation in base-emitter voltage of the failure detecting transistor due to variation in temperature can be counterbalanced with the diode employed, so that the device of the present invention is remarkably stable against possible variation or change in surroundings or temperature.

What is claimed is:

1. An apparatus for automatically detecting and compensating failure of a first lamp normally energizable from a power supply, comprising in combination:

means including a series resistor connecting said first lamp across said power supply and responsive to conduction through said first lamp for creating a voltage drop in said series resistor;

first and second normally non-conductive transistors of opposite polarity; means connecting a main current electrode of said normally non-conductive first transistor between said first lamp and series resistor and responsive to loss of said voltage drop upon failure of said first lamp for turning on said first transistor; bias means connecting the base of said first transistor to said power supply for enabling said main current electrode to control turn on of said first transistor;

means coupling said first transistor to the base of said second transistor and responsive to first transistor conduction for turning on said second transistor;

feedback means coupling a main current electrode of said second transistor to said bias means and responsive to conduction of said second transistor for increasing conductive bias on said first transistor base and accelerating switching of said first and second transistors into conduction; and

a back-up lamp and means coupling same to said sec ond transistor and responsive to conduction of said second transistor for energizing said back-up lamp.

2. An apparatus according to claim I in which said series resistor is of resistance sufficiently small that said voltage drop thereacross is less than the threshold voltage of said first transistor, whereby to minimize power loss in said series resistor during normal current flow through the ignited first lamp.

3. An apparatus according to claim 2 in which said bias means comprises a further resistor and a diode in series between said power supply and said base of said first transistor. said main electrode of said second transistor being connected at least through said further resistor to said power supply to provide positive feedback from said second transistor upon conduction thereof to said first transistor, whereby said series diode and further resistor and said positive feedback provide rapid and positive actuation of said transistors despite the relatively small change in voltage drop across said series resistor upon failure of said first lamp.

4. The apparatus of claim 1 in which the emitter is the electrode of said first transistor connected between said series resistor and first lamp, the collector of said first transistor is direct connected to the base of said second transistor, said bias means comprises a diode and further resistor in series, one end of said bias means being connected to said power supply and the other end being connected to the base of said first transistor and the collector of second transistor, said means coupling said back-up lamp to said second transistor comprising a conductive path connected to the emitter of said second transistor and including a path to circuit ground for said back-up lamp and first lamp and resistance means connecting the base and collector of said second transistor to said circuit ground.

5. An apparatus according to claim 1 in which said bias means comprises a diode and further resistor in series, the first recited coupling means comprising a resistor connecting the first transistor collector to the second transistor base, said feedback means including an additional resistor connecting the second transistor collector to the first transistor base, the remaining said coupling means comprising a third transistor connected in series between said back-up lamp and power supply and having a base electrode coupled to the collector of said second transistor for actuation of said third transistor upon actuation of said second transistor.

6. An apparatus according to claim 1 in which said bias means comprises a further resistor and diode in series between said power supply and said first transistor base, the diode-connected end of said further resistor being connected by said feedback means in series with the main current electrodes of said second transistor and thence through said last mentioned coupling means with said back-up lamp, said last mentioned coupling means comprising a conductive connection between the emitter of said second transistor and said back-up lamp, the first mentioned coupling means comprising a resistor connected between said first transistor collector and said second transistor base.

7. A device for automatically detecting and compensating failure in a lamp normally energizable from a power supply, which comprises:

a first lamp, the failure of which is to be detected and compensated;

a back-up lamp energizable in the place of said first lamp, when said first lamp is burnt out;

a series resistor connected between said power supply and said first lamp;

a PNP transistor having a collector, an emitter connected between said series resistor and said first lamp, and a base connected to said power supply through a bias path including a further resistor and diode in series;

an NPN transistor having a base;

means connecting said base of said NPN transistor to the collector of said PNP transistor for turning on said NPN transistor when the potential at the emitter of said PNP transistor is higher than the potential at the base thereof, said NPN transistor having main current electrodes and means coupling one thereof to said bias path for further lowering the (ill lamp therethrought 9. A device according to claim 7 wherein said diode is connected at its cathode to the base of said PNP transistor and at its anode to said power supply through said further resistor and to the collector of said NPN transistor. 

1. An apparatus for automatically detecting and compensating failure of a first lamp normally energizable from a power supply, comprising in combination: means including a series resistor connecting said first lamp across said power supply and responsive to conduction through said first lamp for creating a voltage drop in said series resistor; first and second normally non-conductive transistors of opposite polarity; means connecting a main current electrode of said normally nonconductive first transistor between said first lamp and series resistor and responsive to loss of said voltage drop upon failure of said first lamp for turning on said first transistor; bias means connecting the base of said first transistor to said power supply for enabling said main current electrode to control turn on of said first transistor; means coupling said first transistor to the base of said second transistor and responsive to first transistor conduction for turning on said second transistor; feedback means coupling a main current electrode of said second transistor to said bias means and responsive to conduction of said second transistor for increasing conductive bias on said first transistor base and accelerating switching of said first and second transistors into conduction; and a back-up lamp and means coupling same to said second transistor and responsive to conduction of said second transistor for energizing said back-up lamp.
 2. An apparatus according to claim 1 in which said series resistor is of resistance sufficiently small that said voltage drop thereacross is less than the threshold voltage of said first transistor, whereby to minimize power loss in said series resistor during normal current flow through the ignited first lamp.
 3. An apparatus according to claim 2 in which said bias means comprises a further resistor and a diode in series between said power supply and said base of said first transistor, said main electrode of said second transistor being connected at least through said further resistor to said power supply to provide positive feedback from said second transistor upon conduction thereof to said first transistor, whereby said series diode and further resistor and said positive feedback provide rapid and positive actuation of said transistors despite the relatively small change in voltage drop across said series resistor upon failure of said first lamp.
 4. The apparatus of claim 1 in which the emitter is the electrode of said first transistor connected between said series resistor and first lamp, the collector of said first transistor is direct connected to the base of said second transistor, said bias means comprises a diode and further resistor in series, one end of said bias means being connected to said power supply and the other end being connected to the base of said first transistor and the collector of second transistor, said means coupling said back-up lamp to said second transistor comprising a conductive path connected to the emitter of said second transistor and including a path to circuit ground for said back-up lamp and first lamp and resistance means connecting the base and collector of said second transistor to said circuit ground.
 5. An apparatus according to claim 1 in which said bias means comprises a diode and further resistor in series, the first recited coupling means comprising a resistor connecting the first transistor collector to the second transistor base, said feedback means including an additional resistor connecting the second transistor collector to the first transistor base, the remaining said coupling means comprising a third transistor connected in series between said back-up lamp and power supply and having a base electrode coupled to the collector of said second transistor for actuation of said third transistor upon actuation of said second transistor.
 6. An apparatus according to claim 1 in which said bias means comprises a further resistor and diode in series between said power supply and said first transistor base, the diode-connected end of said further resistor being connected by said feedback means in series with the main current electrodes of said second transistor and thence through said last mentioned coupling means with said back-up lamp, said last mentioned coupling means comprising a conductive connection between the emitter of said second transistor and said back-up lamp, the first mentioned coupling means comprising a resistor connected between said first transistor collector and said second transistor base.
 7. A device for automatically detecting and compensating failure in a lamp normally energizable from a power supply, which comprises: a first lamp, the failure of which is to be detected and compensated; a back-up lamp energizable in the place of said first lamp, when said first lamp is burnt out; a series resistor connected between said power supply and said first lamp; a PNP transistor having a collector, an emitter connected between said series resistor and said first lamp, and a base connected to said power supply through a bias path including a further resistor and diode in series; an NPN transistor having a base; means connecting said base of said NPN transistor to the collector of said PNP transistor for turning on said NPN transistor when the potential at the emitter of said PNP transistor is higher than the potential at the base thereof, said NPN transistor having main current electrodes and means coupling one thereof to said bias path for further lowering the potential of said PNP transistor base upon conduction of said NPN transistor; means coupling said NPN transistor and back-up lamp and responsive to conduction of said NPN transistor for energizing said back-up lamp.
 8. A device according to claim 7 including an amplification transisTor conductive in response to the turning on of said NPN transistor for energizing said back-up lamp therethrough.
 9. A device according to claim 7 wherein said diode is connected at its cathode to the base of said PNP transistor and at its anode to said power supply through said further resistor and to the collector of said NPN transistor. 